Earthquake FAQ

Current Earthquake Information

I just felt shaking. Was it an earthquake?

Maybe. Or maybe not. A lot of other things besides earthquakes can cause local shaking - trucks driving by, thunder, etc. Check out this map. It displays recent seismic activity in California and is updated every time we detect and locate an earthquake. Also, this page shows a recent list of earthquakes in Northern California. If you felt an earthquake, help us out and report it in the ``Did You Feel It'' section. Earthquakes will feel different depending on the size of the earthquake and your location to the epicenter. If it is a large earthquake and you are close to it, you will feel a sudden bump followed by intense shaking. It will be difficult to stand. Conversely, if there is a small earthquake and you are not as close to it, you may feel a small bump, followed by a few seconds of sharp shakes.

I saw an earthquake on the USGS realtime map, but then it disappeared. What happened?

There are several reasons why earthquakes might appear and then disappear from the USGS realtime earthquake map. Earthquakes first appear on the map in
response to a computer program's handling of triggers at seismic stations throughout California. Then they are reviewed by the seismologist on call at the USGS. In complex situations, sometimes we don't have a clear picture of what is happening until the event is reviewed by a human. For example, there can be false triggers for when the signal for one event gets mixed up in the signal for another earthquake. Incorrect quake markers can also show up when the system triggers on the P wave from a large, more distant earthquake. Other signals that aren't earthquakes can also result in false alarms that have to be removed from the map. For example, our stations sometimes detect sonic booms, space shuttle re-entry, or even very loud thunder in areas where the network is very dense. When the rate of seismicity in a region is extremely high (such as a region experiencing many aftershocks following a large earthquake), the automated system often has problems distinguishing different earthquakes. The computer system may pick up one quake triggering on many stations and list it as two. Or one apparent large quake can resolve into many smaller earthquakes upon inspection.

Earthquakes and the Bay Area

When will an earthquake happen in the Bay Area?

Actually, many earthquakes happen in the Bay Area happen every day. Most of them are too small to feel. On average, there is an earthquake that a few people will feel every 2-3 weeks and one that will be felt by many people every year. Visit this website to see a map of the earthquakes that have occured in the Bay Area in the past 7 days. We cannot predict exactly when the next big earthquake will happen in the Bay Area, or anywhere else; we can only give a probability. Within the next 30 years, the probability is 62% that we will experience an EQ with magnitude 6.7 or greater on one of our faults. Southern California is in a similar boat: Within the next 30 years, there is a 60% chance that people there will have an EQ with magnitude 6.7 or greater.

Why do people always talk about the Hayward Fault specifically as a hazard instead of other Bay Area faults like the San Andreas Fault?

There are two reasons for this. One is the time between quakes on these two faults. The 1868 earthquake on the Hayward Fault was considered the ``Great San Francisco earthquake'' until the San Andreas Fault ruptured in 1906. Before then, we did not know about the hazard of either fault. Our newest scientific studies show that earthquakes on the San Andreas Fault happen about every 200 years, while they happen on average every 140 years on the Hayward Fault. Do the math: 1868 + 140 = 2008, while 1906 + 200 = 2106. The San Andreas Fault appears not to be ``due'' as soon as the Hayward. The second reason is vulnerability: The San Andreas Fault runs along the hills of the Peninsula and is offshore from Pacifica to Stinson Beach. The Hayward Fault passes along the foot of the East Bay Hills, through Fremont, Hayward, Oakland, etc. Many more people live directly on the Hayward Fault than on the San Andreas Fault. In addition, much of the infrastructure that supports the people in the Bay Area crosses the Hayward Fault: water for San Francisco and EBMUD, power lines, freeways, BART ...

The Golden Gate Bridge is currently undergoing a sequence of retrofits, some of which have already been completed. The process has reached the point that the Golden Gate Bridge no longer faces the potential for collapse. Until all the retrofit steps have been completed, the Main Suspension Bridge may be damaged significantly in a major earthquake. Check for updates at the website of the Golden Gate Bridge. The new eastern section of the Bay Bridge, which is expected to open in 2013, is being built to withstand a major earthquake. The retrofit of the western segment has been completed. The Bay Bridge is considered an emergency ``lifeline'' route to be used in disaster response activities. Thus, when completed, it will be able to reopen quickly following an earthquake. For more information, visit http://www.baybridgeinfo.org/faqs.

Earthquake Engineering is a very important field contributing to public safety in the event of an earthquake. Research is always being done on how to make new and old buildings safer. Scientists and engineers at universities around the world are studying the impact of earthquakes on the built environment, with the goal of designing safer structures. For example, engineers at the UC Berkeley Earthquake Engineering Research Center test designs on their shake table. A sampling of those organizations is given below:

If there were to be an earthquake larger than 6.5, would East Bay soil liquify? If so, how many cities would disappear?

Although there are many places in the East Bay that are prone to liquefaction in a large earthquake, this phenomena can't cause entire cities to disappear. In liquefaction, water-saturated soil loses strength when shaking happens due to changes in water pressure between the particles of soil. In a magnitude 6.5 East Bay earthquake, sewers and other underground pipes would break, and buildings might be damaged or collapse, or even tip over and sink partly into the soil. Our airport runways could be ruined, and area roads and freeways would also be damaged. Much of the damage from the magnitude 6.1 Christchurch, New Zealand earthquake of February 22, 2011 was the result of liquefaction.

The Association of Bay Area Governments Earthquake and Hazards Program has published a guide called "The Real Dirt on Liquefaction" that covers the hows and whys of liquefaction, and what could happen to our infrastructure. You can download the report from this webpage, which also hosts liquefaction maps for the region.

Seismic hazard maps are maps that show expected level of shaking in a particular
area due to earthquakes that may occur in the region. These can be used by local
governments for construction projects for new buildings or retrofitting old ones. They can also be used by citizens concerned with seismic dangers. There are many places to find these publicly available resources and a quick Google search will yield many useful results. Here are a couple of good sites:

The Alquist-Priolo (AP) Fault Zoning Act was passed as a response to the February 9, 1971 Mw 6.6 San Fernando earthquake. The act creates special regulatory zones, or earthquake fault zones, on and around active faults throughout California. Additionally, maps for these areas are created and distributed to relevant cities and counties. This will ensure that any construction on or near a fault can take the necessary measures to ensure public safety. The maps are also made available to the public, which can be very useful information for various purposes, such as knowing where a fault lies before buying a house. For more detailed information on the AP Act, and for maps and other resources, visit the State of California Department of Conservation webiste.

Where can I learn more about Bay Area Faults and Significant Bay Area quakes?

We have a lot more information available about Bay Area seismological features.

Most modern single-family wood-frame homes in the Bay Area are safe to
be in during an earthquake. Building safety depends more on building
design, construction type, building material quality, soil conditions
at the property site and less on the number of stories. The exception
to this rule are multi-story buildings with a "soft story", a ground
floor with large openings to accommodate windows or garage doors. If
not properly braced, these openings can reduce the shear strength of
the structure making it vulnerable to strong ground motions. For a
detailed description on how different building types should perform
during an earthquake, please visit the Association of Bay Area
Governments.
Or, click here
to see a full scale
shake table test of a seven story wood-frame condominium building
outfitted with Simpson Strong-Tie products. This test was conducted at
Japan's Earth-Defense shake table located just north of Kobe, Japan.

Do you think San Francisco is going to have a big earthquake soon?

The 2007 Working Group on California Earthquake Probabilities (WGCEP
2007), a multi-disciplinary collaboration of scientists and engineers,
has forecast a 63% chance that an earthquake of magnitude 6.7 or
greater will occur in the San Francisco Bay Area between 2008 and
2038. Given this
high likelihood, you should educate yourself now about steps that can
be taken now to ensure you and your loved ones are prepared for when
the next big earthquake hits. For more information about studies on future earthquakes in the Bay Area, check out our Seismo blog article on probabilities

Which buildings on the UC Berkeley campus are the most earthquake-unsafe?

Since 1997, UC Berkeley has completed or initiated approximately $500
million worth of seismic and related improvements in buildings across
campus. This effort started with the student housing facilities,
followed by all occupied buildings on the central campus that had a
"very poor" seismic rating.

Presently, Memorial Stadium, the campus's most notable seismic
retrofit, is under construction. Information about this project and a
live webcam view of the construction site can accessed from
http://stadium.berkeley.edu/.

Seismic retrofitting is still needed for smaller, rarely occupied
facilities such as the Old Art Gallery and backstage at the Greek
Theater. Work that will further improve the seismic safety of the UC
Berkeley campus is in progress, and the SAFER program continues to
provide a framework that guides campus planning and investment.

Common Myths and Misconceptions

Can small EQ's relieve stress to prevent large ones?

If you look at earthquake statistics in most regions of the world, including California, you will find that for every magnitude 5 earthquake, there are about 10 that have a magnitude of 4, and for each magnitude 4, there are 10 with magnitude 3. Unfortunately, this means there are not enough small earthquakes to relieve enough stress to prevent the large events. In fact, it would take 32 magnitude 5's, 1000 magnitude 4's, or 32,000 magnitude 3's to equal the energy produced in one magnitude 6 event.

During an earthquake, should you head for the doorway?

Only if you live in an old, unreinforced adobe house. In modern homes, doorways are not stronger than any other parts of the house, and the doors in them may swing and injure you. You are safer practicing ``DROP, COVER, AND HOLD ON'' under a sturdy piece of furniture. Every year, institutions and agencies that deal with earthquake hazard encourage everyone to participate in a preparedness drill. Find out more and sign up at http://www.shakeout.org. Another tidbit of information: if the shaking stops before you have had a chance to decide what to do, then you didn't need to do anything. If you need to ``drop, cover, and hold on,'' you'll have plenty of time to decide.

Will California eventually fall off into the ocean?

No. The San Andreas Fault System, which crosses California from the Salton Sea in the southeast to Cape Mendocino in the north, is the boundary between the Pacific Plate and North American Plate. The Pacific Plate is moving northwest with respect to the North American Plate at approximately 46 millimeters, or about 2 inches per year (the rate your fingernails grow). The strike-slip earthquakes on the San Andreas Fault are a result of this plate motion. The plates are moving horizontally past one another, so California is not going to fall into the ocean. However, in the very distant future, Los Angeles and San Francisco may one day be adjacent to one another!

Can the position of the moon or planets affect seismicity?

The question often arises as to whether astronomical events, such as planetary
alignments, can significantly influence the occurrence of earthquakes. The
moon, sun, and other planetary bodies in our solar system influence the earth in
the form of perturbations to the gravitational field. The relative amount of
influence is proportional to the object's mass and inversely proportional to the
cube of its distance from the earth. * The general
idea is that strains in the
earth's crust caused by perturbations in the gravitational field may influence
when an impending earthquake will occur (like the straw that broke the camel's
back). If this were indeed the case, we would expect to see a correlation
between rate at which earthquakes occur and the perturbations to the
gravitational field. The dominant perturbation in the earth's gravitational
field generates the semi-diurnal (12 hour) ocean and solid earth tides which
are primarily caused by the moon (due to its proximity) and the sun (due to its
large mass). No significant correlations have been identified between the
rate of earthquake occurrence and the semi-diurnal tides when using large
earthquake catalogs. There have, however, been some small but significant
correlations reported between the semi-diurnal tides and the rate of occurrence
of aftershocks in some volcanic regions, such as Mammoth Lakes.

The relative influences of object in the solar system, in order of magnitude.
are:

Object

Mass
(Earth=1)

Distance
(Million km)

Relative Influence
(Moon=1)

Moon

0.01228

0.38

1.00

Sun

329390

149

0.45

Venus

0.8073

41

0.000052

Jupiter

314.5

629

0.0000056

Mars

0.1065

79

0.00000096

Mercury

0.0549

91

0.00000033

Saturn

94.07

1277

0.00000020

Uranus

14.40

2720

0.000000004

The combined influence of the rest of the objects in the solar system is less
than 10 billionths of the influence if the moon. The combined influence of
all objects in the solar system, other than the moon and the sun, is at most
0.000059 or only 1/24500 the combined influence of the moon and the sun. Thus,
even when all the planets are lined up, their combined influence is relatively
small.

Besides the dominant semi-diurnal periodicity, there are other significant
periods. Most notably, there is the synodic month (~29.53 days) periodicity
due to the moon's orbit around the earth (relative to the sun) and the 18.5
year periodicity due to the 5 degree inclination of the moons orbit. No
significant correlations between these periods and the rate of occurrence of
earthquakes have been found.

Given the relative influence of a planetary alignment and the lack of
correlation of earthquakes with the dominant gravitational effects, we would
not expect planetary alignments to significantly infleunce either the rate of
occurrence of earthquakes or the relative motion of the tectonic plates. No
significant correlations of earthquakes with planetary alignments have been
found.

The gravitational influence of the other bodies in the solar system is largest
in the vicinity of the earth's equator and smallest near the poles.

There are two types of weather that we experience on earth. One type is space weather. This is weather that comes to us from beyond our own planet, such as solar flares and magnetic storms. These phenomena result in things like the Northern Lights. While this type of weather can influence our communications and other systems, it has never been shown to affect earthquakes. If there were a connection, we would see an increase in earthquakes about every 11 years, which is when solar flares are the most intense, yet we see an even distribution of earthquakes regardless of what the Sun is doing.

The other type of weather is what we are all familiar with, rain, wind, heat, etc. Again, statistically, there is an even distribution of earthquake events throughout all types of weather. However, very large low-pressure systems, such as hurricanes, have been known to cause episodes of fault slip (slow earthquakes), which are not very damaging.

Does the ground open up during and earthquake?

Shallow crevasses can form during earthquake-induced landslides, lateral spreads, or other types of ground failures. Faults, however, do not open up during an earthquake. Movement occurs along the plane of a fault, not perpendicular to it. If faults opened up, no earthquake would occur because there would be no friction to lock them together. (source: USGS)

Can we prevent earthquakes?

There is no way for us to prevent earthquakes. After all, we can't stop the earth's tectonic plates from moving. And, although mining and reservoir activities can, and do, cause earthquakes, we can't realistically cause enough tiny earthquakes to prevent a large one. This is because for each increase in magnitude, the amount of energy released increases 32 times. So, for example, it would take about 33,000 magnitude 3 earthquakes to equal the amount of energy released in a magnitude 6.0 earthquake. (In nature, we only get about 1000 magnitude 3 quakes for every quake of magnitude 6 - not nearly enough to release all that energy.)

What we can prevent are deaths, injuries, and property damage caused by earthquakes. Look around your home for heavy objects that could fall on top of you in an earthquake, and find a better spot for them. Practice Drop, Cover, and Hold On!" in different rooms of your home. Make an emergency kit with first aid supplies, food, water, money, and other things you would need after a quake. Make a family plan for contacting each other. Find out whether your home needs to be retrofitted. Click here for good earthquake preparedness links.

Can animals predict earthquakes?

So far, there has been no conclusive evidence that animals can predict
earthquakes or sense that they are about to occur. Animals frequently
exhibit behavior that we call "strange," so it is likely that at any
given time, someone will be witnessing odd animal behavior, whether or
not an earthquake is imminent. But animals are more sensitive than
people in many ways, so they may start to feel the shaking from an
earthquake before their human friends notice it. This video
shows a dog reacting to
an earthquake before his human companion is aware of it. This
heightened sensitivity is sometimes misinterpreted as a pet predicting
the earthquake.

Earthquakes, Faults, and Plate Tectonics

What is an earthquake?

The term earthquake describes both the sudden slip on a fault and the radiated seismic energy and ground shaking caused by the slip. It also covers ground shaking caused by volcanic or magmatic activity and other sudden movement due to stress changes in the earth.

What is a fault and what are the different types?

A fault is a fracture or zone of fractures between two blocks of rock.
Faults allow the blocks to move relative to each other.
This movement may occur rapidly, in the form of an
earthquake - or may occur slowly, in the form of creep.
Faults may range in length from a few millimeters to
thousands of kilometers. Most faults produce repeated
displacements over geologic time.

During an earthquake, the rock on one side of the fault suddenly slips
with respect to the other. The fault surface can be horizontal
or vertical or some arbitrary angle in between. Earth scientists
use the angle of the fault with respect to the surface (known as
the dip) and the direction of slip along the fault to classify
faults. Faults which move along the direction of the dip plane
are dip-slip faults and described as either normal or reverse, depending on their motion. Faults which move
horizontally are known as strike-slip faults and are
classified as either right-lateral or left-lateral.
Faults which show both dip-slip and strike-slip motion are known
as oblique-slip faults. Great animations of these types of fault motion are available from the IRIS Consortium.

The following definitions are adapted from The Earth by Press and
Siever. This book, and many other good references, are listed
in the our Suggested Reading index.

Normal Fault
A dip-slip fault in which the block above the fault has moved downward
relative to the block below. This type of faulting occurs in response
to extension and is often observed in the Western United States Basin and
Range Province and along oceanic ridge systems.

Reverse Fault
A dip-slip fault in which the upper block, above the fault plane, moves
up and over the lower block. This type of faulting is common in
areas of compression, such as regions where one plate is being subducted
under another as in Japan. When the dip angle is shallow, a reverse
fault is often described as a thrust fault.

Left-Lateral Fault
A strike-slip fault on which the displacement of the far block is to
the left when viewed from either side.

Right-Lateral Fault
A strike-slip fault on which the displacement of the far block is to
the right when viewed from either side. The San Andreas Fault is
an example of a right lateral fault.

Additional information on the types of faults and earthquakes
can be found in our FAQ on "What are
those beachball figures?"

Why do the plates move?

The inside of the Earth is hot because of radioactive rocks inside the Earth and because of heat leftover from the immense pressures of the Earth's formation. So the Earth is cooling off. As the Earth cools, hot rock within the mantle is very slowly coming up to the surface, while cold rock is very slowly sinking down towards the core. This is called convection.

Meanwhile, the plates on the Earth's surface are moving around in different ways. In some places, like right under Chile, one plate is being pushed (subducted) under another. The part of the plate that has gone under the other plate is sinking into the mantle, and it's very heavy. The sinking plate is part of the downward flow of hot rock in this convection of the mantle, and we know the force of all that sinking rock is an important part of what causes the plates to move. We have used seismic waves to make pictures of the mantle, so we also know that there are areas where hot rock is coming up to the surface called mantle plumes. Hot rock in the mantle is also (again, very slowly) moving horizontally between places where it goes up and places where it goes down. Does it drag the plates along with it? Scientists don't know. So our picture of the forces driving the plates is not complete.

What are aftershocks and foreshocks?

Aftershocks and foreshocks are related terms describing earthquakes that happen before or after a "mainshock", that is to say, an earthquake. Foreshocks are earthquakes that occur in the same location as a mainshock, but before the earthquake happens, whereas aftershocks are smaller earthquakes that occur after the mainshock in the same area, but not necessarily the same exact location. Aftershocks generally decrease with time after the main event and are much less common in deep earthquakes.

How are tsunamis and volcanoes affected by earthquakes?

Volcanoes and earthquakes are almost never related. They very rarely have anything to do with one another. The USGS has a nice write up about their relationship here. On the
other hand, tsunamis are directly cause by earthquakes that are located in the ocean. The vertical deformation resulting from an earthquake rupture gives rise to the tsunami, which can be very small and unnoticeable, or very large and can cause much destruction and loss of life.

Why can't we average together data from the GPS units in our mobile phones and laptops to measure plate motions?

The main problem with crowdsourcing GPS is that it works really badly indoors because of the poor (sometimes non-existent) sky view, which is where most people spend most of their time. But even outdoors, the 30m
accuracy provided by most mobile phones and laptops means that we would need prohibitively high numbers of data points* to get mm level accuracy, which is what we need for measuring plate rates. (For geodetic grade GPS stations, we use the signal from the GPS satellites in a fundamentally different way from the way handheld devices use it, in order to get mm level accuracy at each station.) Moreover, the rates we are measuring are so slow that they could not be measured over the course of a single day (one would need sub-mm accuracy for that.) So people would need to put their laptops back in the same place, within a mm, every day in order to track velocity over a longer time.

Secondly, there is a geophysical problem with using widely distributed GPS signals to track plate motions. Strain accumulation around faults causes the areas near them to move at different rates
than the total plate rate. If you think of it like pulling on a rubber band, then your hands are two tectonic plates moving at constant rates away from each other, but various points on the rubber band itself are moving more slowly, and, in fact, the center of the rubber band isn't moving at all. Until the rubber band breaks (earthquake), at which point those parts of the rubber band that were moving more slowly suddenly move very quickly and catch up with your hands (the tectonic plates). In geodetic studies of plate motion, data from GPS sites near faults are usually excluded in order to prevent these altered rates from affecting the estimate of total plate motion (of course these data are very important for measuring the strain accumulation itself). In the US this means that data from much of the western US is excluded; in Europe much of Mediterranean region would have to be excluded. And in areas like Asia with lots of microplates, finding enough data away from plate boundaries would be even harder.

*Averaging reduces uncertainty by a factor of 1/square root (N), where N is the number of values being averaged together. So we would need on the order of 900 million data points to get mm level average uncertainty from 30m uncertainty data. This could mean that we would need 900 million people for each tectonic plate.

What is intensity?

Intensity describes how much the ground shakes during an earthquake. For a given earthquake, intensity is different from one place to another depending on the shaking, but the quake has only one magnitude. Factors that determine intensity at a specific spot include the rock and soil under it and how far it is from the epicenter of a quake. The intensity is usually given on the Modified Mercalli Intensity (MMI) Scale, a qualitative measure based on groups of standard observations, such as "Difficult to stand" or "Shutters, pictures move". To distinguish it from magnitude, intensity is assigned Roman numerals I-XII, with XII being the highest. If you recently experienced an earthquake and want to contribute to an intensity study, fill out a felt report.

Why are there aftershocks?

Imagine you and a friend sliding a large wooden dresser to a new
location across a wood or tile floor. After it's in place, you may hear
small popping or squeaking noises coming from it as it settles. In the same way that moving your large wooden dresser disrupts
the pressure each piece is putting on other pieces at the joints, and
the pressure that the legs are putting on the floor, an earthquake
causes changes in the details of the stress on the fault where the
slip occurred and nearby.

Before an earthquake, stress builds up on a fault, and in a major
earthquake, stress is released, but it's more correct to think about a
more complex "stress field" of varying stress all around the fault.
When a big earthquake happens, the stress field around the earthquake
changes in response. In some areas, the stress increases, and this can
set off the smaller earthquakes we know as aftershocks. (If the next
earthquakes is larger, it becomes the mainshock and the previous
earthquake becomes a foreshock!)

How many aftershocks can there be in one day?

Earthquakes, large and very small, are happening all over the world,
all the time. For an illustration of this, click here to see
earthquakes that have been located in California and Nevada in the
past 7 days. We don't know exactly how small an earthquake has to be before it
can't trigger any aftershocks, but it is well below the magnitude
someone could feel (roughly magnitude 2).

Aftershocks and all other earthquakes follow an empirical rule called
Gutenberg-Richter Law describing the number of earthquakes of
different sizes. This law is a logarithmic relationship* that
basically tells us that each time we go down a unit in magnitude, we
should expect to see 10 times as many earthquakes. So for each
earthquake we see of magnitude 5, we should expect to see, very
roughly, 10 4's, 100 3's, 1000 2's, etc. The Gutenberg-Richter
relationship may break down for the tiniest of earthquakes * (so
small that they have negative magnitude), but even so, the number of
teeny-tiny aftershocks generated by all of the tiny earthquakes that
occur - most of which are so small and remote that nobody even feels
them - has to be absolutely staggering.

So let's talk instead about how many aftershocks we could expect to see in a day
following one really large earthquake. Another law, Omori's Law*, describes the
number of aftershocks we expect to see over time following an earthquake of a
given magnitude. Omori's Law tells us that there will be lots of aftershocks
immediately following an earthquake, and that as time passes, the number of
aftershocks will decay exponentially. So the first day after a major earthquake
is when we would expect to see by far the largest number of aftershocks.
Although the basic mathematical relationship in Omori's law doesn't change
broadly, the numbers (the constants) used vary region to region, and even from
sequence to sequence. But in 1989, two scientists* hammered out appropriate
constants that seemed to work for California and put together Omori's Law and
the Gutenberg-Richter relationship into a function for the expected number of
aftershocks over a given time period after a California earthquake of some
magnitude. As statistical measures, Omori's law, and the more detailed
aftershock production rate rule constructed by Reasenberg and Jones, are not
meant to predict exactly how many aftershocks of a given magnitude we will see
over a given time interval, any more than knowing heart attack statistics could
tell you exactly how many people will show up at a given hospital with one on a
particular day.

The Reasenberg and Jones relationship is a mouthful:
rate(t,M)=10^(-1.67+0.91(Mm-M))*(t+0.05)-1.08, where t is time in
days, Mm is the magnitude of the mainshock, and M is the magnitude of
aftershocks that we are looking at. So, for a magnitude 8 event, if we
want to look only at magnitude 2 plus events, Mm-M would be 6.
Plugging in the numbers, for a magnitude 8 earthquake, about the
biggest that we could experience in California, we get an expected
5849 M2+ aftershocks, 720 M3+ aftershocks, 89 M4+ aftershocks, 11 M5+
aftershocks, and 1 M6+ aftershock. The vast majority of these
thousands of aftershocks are between magnitude 2 and 3, just at the
level that someone in the vicinity might start to feel them and
certainly not big enough to cause any damage. However, though each of
these aftershocks could potentially be felt by someone standing
nearby, you certainly wouldn't feel all, or even most of them, which
for a large earthquake is considerable. The 1857 M 7.9 Fort Tejon
earthquake, in the running for California's largest earthquake in
written history, ruptured over 350 km, which translates to an area for
aftershocks spanning about 12000 square miles!

The largest earthquake ever recorded in the world was not of magnitude 8,
however, but magnitude 9.5, in Chile in 1960. We can't use Reasenberg and Jones'
California numbers for Chile, but this size of an event would probably generate
more than 100,000 aftershocks of magnitude 2+ or above on the first day after
the quake.

(1) log N = a-bM, where N is number, M is magnitude, and a and b are
constants that vary by region.

We thought that this was such an important topic that we wrote this blog
about it in September 2008.

Measuring Earthquakes

How do seismologists measure earthquakes?

In the United States, large-scale seismological networks are generally
run by federal agencies (such as the USGS,
the Bureau of Reclamation, and the Department
of Energy), by state agencies, and by public and private universities.
These networks are designed to monitor earthquake
activity and to provide data for research into
earth science problems. The
Advanced National Seismic System
is an organization of institutions involved in
seismic monitoring with the goal of
coordinating efforts to record and analyse
seismic data.

For example, UC Berkeley operates a seismic network
in northern and central
California for the purposes of monitoring seismic
activity and furthering earthquake research.
The seismographic instrumentation used in the
Berkeley Digital Seismic Network (BDSN)
includes broadband seismometers to sense weak ground motions and
accelerometers to sense strong ground motions. Both types of
sensors utilize force-feedback circuitry to determine the overall
response, linearity, stability, and dynamic range of the sensors.
UC Berkeley operates several different broadband seismometers and
accelerometers in order to cover the widest range
of frequencies and ground motions:

Typical broadband seismometers are capable and responsive to weak
ground motions ranging in frequency from the semi-diurnal gravitational
tides at ~23 microHz to ~5-40 Hz (depending on sensor bandwidth).

The primary limitations at the low-frequency end of the spectrum are the
seismic background noise level, the instrumental noise level, and the
thermal stability of the sensor and the data logger. At the best BDSN
sites, the semi-diurnal gravitational tide signal is readily apparent
in the raw data while at the noisier sites it is not resolvable.

The primary limitations at the high-frequency end of the spectrum are the
digital sampling rate, the instrumental noise level, and the attenuation
and scattering of the surface weathered layer (the upper ~100 meters of
the crust). The best BDSN stations, with sensors installed in boreholes,
typically see 200+ Hz signals generated by local earthquakes. The best
BDSN stations, with sensors installed on the surface, typically do not
register significant energy at frequencies above ~30 Hz.

Seismic frequency bands of interest:

Gravitational tides

~0 Hz to ~70 microHz
(periods of 4+ hours)

Earth's eigenvibrations

~0.3 mHz to ~0.1 Hz

Surface wave analysis

~2 mHz to ~2 Hz

Regional earthquakes

~10 mHz to ~10 Hz

Local earthquakes

~10 mHz to ~400+ Hz

Strong motion

~0.05 Hz to ~10 Hz (frequency band which usually causes structural
damage during strong ground shaking)

The capabilities of the modern generation of seismic instrumentation was
driven by the needs of pure research and made possible by the advent of
large scale integrated circuit technology. As our knowledge of earth
structure and our ability to model the earth at higher frequencies
improves, accurate recordings at yet higher frequencies will become
useful. At lower frequencies, primarily associated with secular
deformation of the earth's crust, data are provided by continuously
operating Global Positioning System (GPS) receivers. The UC Berkeley
Seismographic Station is collaborating with a number of agencies in
northern California to form the
Bay Area Regional
Deformation (BARD) Network to monitor crustal deformation as well as
seismic activity. Many of the BARD GPS receivers are co-located with
BDSN instrumentation.

Information on the installation of the seismic instrumentation is available from the
BDSN Installation Guide. Finally, if you are interested in seeing what an earthquake looks like, here
are some examples of earthquakes recorded by the BDSN.

What are those beachball figures?

Figure courtesy of David Oppenheimer of the USGS.

In addition to determining the location and magnitude of earthquakes,
seismologists are now routinely determining the "fault plane" solutions
or "focal mechanisms" of events. A fault plane solution illustrates
the direction of slip and the orientation of the fault during the
earthquake. These solutions, which are displayed in lower-hemisphere
projects frequently described as "beachballs", can be determined
from the first-motion of P-waves and from the inversion of
seismic waveforms. These figures help identify the type of earthquake
rupture: strike-slip, normal, or thrust.

Strike-slip earthquakes are typical of the San Andreas fault zone,
which forms part of the boundary between the North American and
Pacific plates.

Normal earthquakes are associated with extension, particularly
with formation of plates at mid-ocean ridges.

Thrust or reverse earthquakes are associated with compression, particularly
with the subduction of one plate under another as in Japan.

In order to study earthquakes, scientists deploy
seismometers to measure ground motion.
Seismograms are recordings of ground motion
as a function of time and are the basic data which
seismologists use to study the waves generated
by earthquakes. These data are used to study the
earthquakes themselves and to learn more about the
structure of the Earth.

We have gathered several examples of earthquake recordings
to illustrate the wide variety of motion. These
data are derived from the
Berkeley Digital Seismic Network, an array of
broadband, high-dynamic range instruments in northern
and central California. This network is operated by
the
UC Berkeley
Seismological Laboratory for earthquake monitoring
and research.

Seismologists generally describe earthquakes as local,
regional, or teleseismic. These terms refer
to distance from the earthquake to the recording
instrument. For example, when the Berkeley
Seismographic Station refers to a local earthquake,
we mean one which has occurred within Northern California.
An example of a regional earthquake might be an
event in Southern California, Nevada, Utah, Oregon,
or Washington. Teleseismic events are those
which occur at great distances, such as earthquakes
in Japan, Tonga, or Iceland.

Local and regional earthquakes are dominated by
crustal waves, i.e., by waves which propagate
through the crust. At greater distances, the seismic
wavefield is dominated by waves which sample the
body of the earth
- the upper mantle, the lower mantle, and the core.

The seismic recording instruments of the BDSN are capable
of "seeing" earthquakes around the globe.
In this example, we will illustrate the variations in
waveforms among these types of earthquakes.

Seismologists have several different methods for determining the size of an earthquake - some based on body waves (which travel deep within the structure of the earth), some based on surface waves (which primarily travel along the uppermost layers of the earth), and some based on completely different methodologies.

Here is a brief description of five of the most common methodologies.

ML - "Local Magnitude" determined for local earthquakes (usually 600 km, or less from the recording station), originally developed by Charles Richter circa 1935 for classification of earthquakes in southern California. ML is defined as

ML = log(a) - log(ao)

where a is the maximum trace amplitude recorded by a standard instrument (the Wood Anderson Torsion seismometer) at a given distance and ao is amplitude for an earthquake of zero magnitude at the same distance. ML has been used most successfully in California, although it is in use in some other regions as well.

Md - "Duration Magnitude" is based on the length of time (starting from the initial P-wave arrival) the seismic wavetrain takes to diminish to 10% or less of its maximum recorded value. Md is mostly used for assigning magnitudes to small earthquakes. In Northern California, it is the preferred type of magnitude for earthquakes of about magnitude 3.0 or less.

where A is maximum displacement, T is the period of the displacement, and s is a correction term for the distance of the station and the depth of the earthquake. Ms was developed by Gutenberg and Richter in 1936 as an extension to local magnitude at greater distances.

mb - "Body Wave Magnitude" which uses the amplitude of the P-wave train, the first arriving body wave, in the magnitude calculation. It is used at teleseismic distances from 16 to about 100 degrees, where this waveform starts to graze and then enter the core of the earth, changing its character. mb is defined in analogous fashion to Ms, with different correction factors.

Each of these magnitudes uses different parts of the seismogram over a different range of frequencies. While effort has been made to calibrate these scales so that they agree with one another, their definitions were limited by the type of instrumentation which existed during their development. For example, ML begins to "saturate" around magnitude 6.5. That is, ML does not properly estimate the size of larger events. In response to this, a new magnitude scale has been developed:

Mw - "Moment Magnitude" is the latest concept in magnitude determination. Unlike the other methods above, which are all based on the maximum amplitude of ground movement at the station, Mw is based on the seismic moment at the source, or hypocenter, of the earthquake. It may be calculated for local earthquakes all the way out to events occurring half way around the world. These are typically determined for local events of about 3.5 and larger magnitude, and teleseismic events of about magnitude 5.5. Smaller events typically don't generate enough energy to provide a sufficently strong signal to perform the determination.

At what magnitude does damage begin to occur?

From the USGS: 'It isn't that simple. There is not one magnitude above which damage will occur. It also depends on other variables, such as the the distance from the earthquake, what type of soil you are on, etc. That being said, damage does not usually occur until the earthquake magnitude reaches somewhere above 4 or 5.'

Do we have a warning system like the one in Japan?

Members of the California Integrated Seismic Network (CISN) are in the
testing stage of an end-to-end early warning system similar to the one
in Japan. Dr. Richard Allen of the Berkeley Seismological Laboratory
has pioneered several methods that make earthquake early warning
possible and is one of the project's principal
contributors. Information on the recent earthquake early warning
summit that was held here at UC Berkeley can be found
here
and other information related to earthquake early warning
can be found on Dr. Allen's
homepage.

When did a person come up with an earthquake detector?

Chang Heng invented the world's first seismoscope around 132 AD. A
seismoscope is a device that measures the direction of the epicentral
location of an earthquake but does not provide measurements of shaking
intensity or duration. For a detailed description and picutre of
Chang Heng's Dragon Jar, please visit:
http://earthquake.usgs.gov/learn/eqmonitoring/eq-mon-6.php#earliest.

In 1880, Sir James Alfred Ewing, Thomas Gray and John Milne, all
British scientists working in Japan, began to study earthquakes. They
founded the Seismological Society of Japan, and the society funded the
invention of seismographs. One of their designs, the Ewing Duplex
Pendulum, was installed by Edward S. Holden, the first President of
the James Lick Observatory, in 1887 at both the James Lick and Student
Observatories. These installations, among others, would later record
the Great San Francisco Earthquake in 1906.

What is the purpose of measuring earthquakes?

Primarily, earthquakes are recorded to collect an objective catalog of
seismicity. Communities use this information to assess their seismic
hazard and mitigate their risk. However, the data produced by these
seismic networks have other applications. In one instance, the data
are monitored to enforce the Comprehensive Test Ban Treaty,
a ban on the testing of nuclear
weapons. Seismologists help to enforce this ban by identifying
distinct traces in the seismic record that can only be caused by a
nuclear weapon detonation. In another application, seismologists
observe the travel times
of the different
wave phases emitted from a seismic source (earthquake) to model the
structure and composition of the earth's interior. Such observations
led to the discovery of the Earth's core in 1906.

What are the differences between explosions and earthquakes?

Both earthquakes and nuclear tests can rapidly release a large amount of energy. The energy source for small yield (typically less than 50 kilotons) thermonuclear devices is the splitting of heavy radioactive isotopes, whereas the energy source for an earthquake is tectonic strain accumulated by the relative motion of Earth's tectonic plates which is driven by mantle heat flow in the presence of the earth's gravitational field.

In a nuclear test, all of the energy is suddenly (within milliseconds) released in the form of heat from a relatively small volume surrounding the thermo-nuclear device. The tremendous heat causes rapid expansion of a spherical cavity, which in turn generates seismic waves. The heat gradually conducts away from the cavity into the surrounding rock. However, rock is a poor conductor of heat so it can take many years for the thermal signature of the thermonuclear explosion to subside and the increase in the surface temperature above the explosion is insignificant. Nuclear tests are also very shallow sources with the depth of burial generally less than a few hundred meters (the depth of burial is typically proportional to the cube root of the expected yield). The estimated yields of the larger Indian and Pakistani tests are approximately 2-40 kilotons.

In a large earthquake, the elastic strain energy stored in the Earth's crust is released, within a few seconds to a few tens of seconds, by rupture along a fault and the strain energy is released from a relatively large volume of rock surrounding the fault rupture. For example, the (5/30/98 at 06:22:28 UT) magnitude 6.5 earthquake in Afghanistan (37.4 N, 70.0 E), had a source duration of about 5 seconds and an estimated source volume of order 4000 cubic kilometers. This earthquake also had a focal depth of 18 km. The energy release is equivalent to a 2000 kiloton nuclear explosion.

Can nuclear explosions cause earthquakes?

On January 19, 1968, a thermonuclear test, codenamed Faultless, took place in the Central Nevada Supplemental Test Area. The codename turned out to be a poor choice of words because a fresh fault rupture some 1200 meters long was produced. Seismographic records showed that the seismic waves produced by the fault movement were much less energetic than those produced directly by the nuclear explosion.

The possibility of large Nevada Test Site nuclear explosions triggering damaging earthquakes in California was publicly raised in 1969. As a test of this possibility, rate of earthquake occurrence in northern California (magnitude 3.5 and larger) and the known times of the six largest thermonuclear tests (1965-1969) were plotted and it was obvious that no peaks in the seismicity occur at the times of the explosions. This is in agreement with theoretical calculations that transient strain from underground thermonuclear explosions is not sufficiently large to trigger fault rupture at distances beyond a few tens of kilometers from the shot point.

The Indian and Pakistani nuclear test sites are approximately 1000 km from the May 30, 1998 Afghanistan earthquake epicenter. The question that has been asked is whether or not the occurrence of these nuclear tests influenced the occurrence of the large earthquake in Afghanistan. The most direct cause-effect relationship is that the passage of the seismic waves, generated by the thermonuclear explosion, through the epicentral region in Afghanistan somehow triggered the earthquake. For example, following the occurrence of the magnitude 7.3 Landers earthquake in southern California on June 28, 1992, the rate of seismicity in several seismically active regions in the western US, as far as 1250 km from the epicenter, abruptly increased coincident with the passage of the earthquake generated seismic wavefield through each site. The abrupt increases in seismicity occurred primarily in regions of geothermal activity and recent volcanism. The mechanism by which this occurred remains unknown.

The Afghanistan earthquake occurred at 06:22:28 UT on May 30, 1998 and the thermonuclear test most closely associated in time occurred at 06:55 UT or after the occurrence of the earthquake. The other nuclear tests occurred 2-20 days before the earthquake. The elastic strains induced in the epicentral region by the passage of the seismic wavefield generated by the largest of the nuclear tests, the May 11 Indian test with an estimated yield of 40 kilotons, is about 100 times smaller than the strains induced by the Earth's semi-diurnal (12 hour) tides that are produced by the gravitational fields of the Moon and the Sun. If small nuclear tests could trigger an earthquake at a distance of 1000 km, equivalent-sized earthquakes, which occur globally at a rate of several per day, would also be expected to trigger earthquakes. No such triggering has been observed. Thus there is no evidence of a causal connection between the nuclear testing and the large earthquake in Afghanistan and it is pure coincidence that they occurred near in time and location.

How can I make my own seismometer?

There are many ways to create your own seismometer that will allow you to view and record seismic waves from your very own home! It is relatively easy to acquire all of the necessary materials required and you can be looking at earthquakes in no time. Scientific American has published two articles on this topic:

"Seismograph Plans: How to build a simple seismograph to record earthquake waves at home", 241, July 1979

"The New Backyard Seismology", 100, April 1996

In addition, there are many local groups of amateur seismologists that participate in public seismic networks. One such network is the Redwood City public seismic network which has a website that has lots of information about how to create your own seismometer.

For educators looking to teach students about seismology, here is a great website to help you get started and with lots of great info! http://www.iris.edu/hq/sis

Historic Earthquakes, and Earthquake Statistics

Where can I find photographs of earthquake damage?

The best collection of damage photographs has been compiled by
the UC Berkeley Earthquake Engineering Research Center (EERC). Karl
Steinbrugge donated his collection of earthquake slides and
photographs to the EERC in 1992. Other slides and photographs from
EERC faculty and staff are collected in these archives as well.
The EERC library provides copies
of the slides and photographs for use in teaching and research for
a minimal cost and online access to the digitized slide collection
is available. This collection includes specialized sets of slides for
the 1994 Northridge and 1995 Kobe earthquakes.

The National Geophysical Data Center of NOAA maintains a collection of
photographs for a variety of geologic hazards, including earthquakes. Most
of their collection is available as slide sets, although a
sampler of their earthquake photographs is available online and includes
images of the 1906 San Francisco and 1989 Loma Prieta earthquakes.

The Library of Congress is creating a National Digital library, comprised
of prints and photos, documents, motion pictures, and sound recordings.
This online collection is searchable and contains a number of images
from the 1906 San Francisco earthquake.

The UC Berkeley Geography Department GeoImages project is
putting an interesting collection of geography slides on the
Web. The Images of the California Environment chapter
has some interesting photographs related to earthquakes.

This question was submitted to us by a 4th grade class in Southern California.
We thought it was a great question and have made it into a FAQ!

Using the ANSS
earthquake catalog, we searched California for all earthquakes from 1990-2011 having a magnitude greater than 1.0. We found 558434 earthquakes! Thus,
on an average year, approximately 25,383 earthquakes are recorded and analyzed.
Then the rate of earthquake occurrence in California is:

Time Number
of earthquakes
Year 25,383
Month 2115
Week 488
Day 70

Thus we record and analyze about 70 earthquakes per day on average. Most of
the analysis is now done by automated computer algorithms so that seismologists
no longer need to manually determine the location and magnitude of each
earthquake. In California, earthquake monitoring is the shared
responsibility of UC Berkeley, Caltech, and the United States Geological
Survey.

As a function of magnitude, the number of events analyzed per year is:

where ">=" means "greater than or equal to". Note that the above table is
not reliable at magnitudes above 6 because the 10 year seismicity sample is
not sufficiently long to include a lot of magnitude 6+ earthquakes. In
generating the above table, we assumed that the rate at which earthquakes
occur does not vary with time. Large aftershock sequences violate this
assumption.

The threshold at which people report feeling an earthquake is approximately
magnitude 2 (under ideal conditions, ie, not moving and in the immediate
vicinity of the epicenter). The threshold at which some damage is reported
(such as broken windows and objects knocked off shelves) is approximately
magnitude 4. The threshold at which damage to weak structures (unreinforced
masonary) occurs is approximately magnitude 5.5.

Was 1999 an unusual year for earthquakes?

1999 was an exciting year for earthquakes. In the latter half of the year, events in Turkey, Taiwan, Mexico, and California all captured headlines. The figure below shows 1999's 22 earthquakes of magnitude 7.0 and higher recorded globally. The size of the symbol reflects its magnitude, while the color and shape indicate its depth below the surface. Seismologists generally classify earthquakes as shallow (events with depths less than 70 km), intermediate (events between 70 and 300 km) and deep (events with depths greater than 300 km).

While it may seem as if Mother Nature was conspiring against us that year, the number of large events (22) was comparable to the average number. The plot below shows the number of earthquakes of magnitude 7 or higher each year for 1899-1999. It ranges from a minimum of 6 (1986) to a maximum of 41 (1943). These data are taken from the USGS National Earthquake Information Center (NEIC). What we saw in 1999 was more events occurring in populated areas. Of these 22 events, most of them were not damaging.

What kind of earthquake was the 1906 San Francisco earthquake?

The 1906 San Francisco earthquake occurred on the San Andreas Fault, a place where two plates are sliding past each other in a motion known as "strike-slip. " The rock where the city of San Francisco is now was down in Southern California 20 million years ago and will continue to move relative to the rest of North America. (Click here to see a video showing the plate tectonic history of Southern California, with San Francisco moving north over the past 20 million years.) The 1906 earthquake measured between magnitude 7.7 and 7.9, making it a major earthquake, but not the largest ever recorded. This honor goes to the 1960 magnitude 9.5 earthquake in Chile. The world's largest earthquakes occur in subduction zones, plate boundaries where one plate is pushing under another.

What was the worst quake ever?

According to the USGS, The earthquake that occurred on January 23,
1556 near Huaxian, Shaanxi (formerly Shensi), China could be
considered the worst ever because it caused more casualties than any
other earthquake in recorded history, with 830,000. The table below
compares this earthquake to other, more recent, large events.

Looking at this table, we see a well defined trend illustrating how
earthquake damage is worse in poorer regions than in more developed
countries like Japan and Chile. ( Most of the casualties of the Great
Tohuku Earthquake in Japan were due the tsunami.)

Where can I find lists of deadly earthquakes?

The USGS has a great page with lots of list that are searchable based on different criteria. It is full of earthquake top 10 lists.

Today in Earthquake History

The USGS has a neat page that presents a historical earthquake that occured on the present day. You can even view historical earthquakes on a day of your choosing!
http://earthquake.usgs.gov/learn/today/